Stop the Prose Gate Reading a Bash Parameter Expansion as a Comment

.github/actions/prose-gate/prose_lint.py

@@ -1770,6 +1770,21 @@ def python_comments(raw: str) -> list[Comment] | None:
     return out
 
 
+PARAMETER_HASH = ("$", "${")
+
+
+def line_marker(line: str, masked: str, marker: str, pos: int) -> int:
+    """Where the first line comment marker at or after `pos` sits, or -1 when there is none.
+
+    A `#` right after `$` or `${` names a parameter, as in `$#` or `${#items[@]}`, not a comment.
+    The prefix is read off the raw line, since an escaped `$` is blanked in the masked one.
+    """
+    where = masked.find(marker, pos)
+    while marker == "#" and where >= 0 and line[:where].endswith(PARAMETER_HASH):
+        where = masked.find(marker, where + 1)
+    return where
+
+
 def extracted_comments(path: Path, lines: list[str], spec: Syntax | None = None) -> list[Comment]:
     """Every comment in the file as (line, text, starts-the-line), for any syntax the fleet uses.
 
@@ -1837,7 +1852,7 @@ def extracted_comments(path: Path, lines: list[str], spec: Syntax | None = None)
             found: str | tuple[str, str] | None = None
             at = len(line)
             for marker in spec["line"]:
-                where = masked.find(marker, pos)
+                where = line_marker(line, masked, marker, pos)
                 if 0 <= where < at:
                     at, found = where, marker
             for opener, closer in spec["block"]:

scripts/tests/test_prose_lint.py

@@ -776,6 +776,35 @@ def test_only_a_c_style_continuation_loses_its_leading_asterisk(self) -> None:
                     [(c.line, c.body, c.leading) for c in found],
                 )
 
+    def test_a_parameter_expansion_hash_opens_no_comment(self) -> None:
+        """A `#` after `$` or `${` is bash naming a count, so neither spelling reads as a comment.
+
+        A real comment on the same kind of line is still found, leading or trailing.
+        """
+        for text in (
+            "            count=${#items[@]}",
+            '            echo "::error::${#items[@]} open issues. Close them by hand."',
+            "            echo $#",
+        ):
+            with self.subTest(line=text):
+                self.assertEqual([], prose_lint.extracted_comments(Path("x.yml"), [text]))
+        self.assertEqual(
+            [],
+            self.flag(
+                "x.yml",
+                'run: |\n  echo "::error::${#items[@]} open issues. Close them by hand."\n',
+            ),
+        )
+        for text, leading in (
+            ("            count=${#items[@]} # Count the open items.", False),
+            ("            # Count the open items.", True),
+        ):
+            with self.subTest(line=text):
+                found = prose_lint.extracted_comments(Path("x.yml"), [text])
+                self.assertEqual(
+                    [("Count the open items.", leading)], [(c.body, c.leading) for c in found]
+                )
+
     def test_a_format_with_no_comment_syntax_is_skipped(self) -> None:
         for name in ("a.lock", "a.csv", "a.txt"):
             with self.subTest(file=name):